Colorado killed a Calgary power play after Ian Cole was penalized for delay of game at 8:01 of overtime.
"It's tough," said Flames goalie Mike Smith, who made 49 saves. "We get a power play, we get a chance in overtime, and the goalie makes a good save, and they bury one of their chances. That's the way it goes, and the series keeps moving forward.
"Cross-seam pass to Rantanen, and he's got an unbelievable release. Buried it."
Rantanen redirected a pass from Nathan MacKinnon from the top of the left circle at the top of the crease to tie the game.
"Not great on power play before that, and we kind of decided we had to be the difference-maker," Rantanen said. "Nate made an unbelievable pass to me at the net, and it went in."